Thank you for your question. I look forward to working with you to provide you the information you are seeking.
Whenever a service is performed by, in this case, a mechanic, even if it is to get an estimate, if work was required to even find out what the problem was, whether or not they could actually repair it or whether or not you decided to go ahead with the repairs, they are entitled to compensation for the time spent doing the work. In the law and courts, this is called the "equitable theory of quantum meruit" which is a court and legal theory that people are entitled to be paid "for the work done." As it does take time away from other work and it does take more than actually looking at the car, because the transmission had to be pulled apart to see what was actually wrong, the mechanic would legally be entitled to compensation for his time in trying to find out what was wrong. Most mechanics will indeed charge for something like this because getting a transmission out for inspection takes considerable time and effort as does putting it back in.
I truly aim to please you as a customer, but please keep in mind that I do not know what you already know or don't know, or with what you need help, unless you tell me. Please consider that I am answering the question or question that is posed in your posting based upon my reading of your post and sometimes misunderstandings can occur. If I did not answer the question you thought you were asking, please respond with the specific question you wanted answered.
Kindly remember the ONLY WAY experts receive any credit at all for spending time with customers is if you click on OK, GOOD or EXCELLENT SERVICE even though you have made a deposit or are a subscription customer. YOU MUST COMPLETE THE RATING FOR THE EXPERT TO RECEIVE ANY CREDIT, if not the site keeps your money on deposit.
Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous.